Conventional methods for homodyne detection include the intradyne method and a carrier method. In the intradyne method, digital signal processing may be required to recover and detect the signal due to variances between the local oscillator and the incoming data signal. The carrier method can include transmitting a carrier signal along with the data signal. In the carrier method, the carrier signal occupies some part of the spectrum and polarization state, and suffers from fiber loss and can accumulate phase noise.
Optical homodyne detection provides better performance and is more sensitive than optical heterodyne detection. However, homodyne-systems require that the local oscillator have the same frequency and phase as the incoming data signal, i.e., the data signal and local oscillator are equal and locked to each other.
A previous approach for carrier recovery includes transmitting a carrier signal along with the data signal. With this approach, the carrier: (a) occupies some part of the spectrum and polarization state, and (b) suffers from fiber loss and can accumulate phase noise. Another approach is to have a local laser oscillator in the receiver, where a phase locked loop (PLL) and signal processing algorithms ensure the locking of the local laser to the same frequency and phase. However, this tends to be fairly complex and requires time to lock. Additionally, some optical methods can recover the carrier of an incoming data signal using nonlinear processing, but these methods typically require an optical feedback loop for stabilization.